This paper presents a novel fuzzy control scheme applied to shunt active power filters for harmonic and reactive power compensation. A TSK type fuzzy logic controller is proposed for APF reference current generation. To control the maximum switching frequency of the converter within limit a novel fuzzy hysteresis band current controller is used. The band height, based on fuzzy control principle is changed with the value of supply voltage and slope of reference current

1. ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 02, June 2011
A Novel Fuzzy Variable-Band Hysteresis Current
Controller For Shunt Active Power Filters
D. A. Gadanayak, Dr. P. C. Panda, Senior Member IEEE,
Electrical Engineering Department,
Electrical Engineering Department,
National Institute of Technology,
National Institute of Technology,
Rourkela, India
Rourkela, India
e-mail: godofsilicon@gmail.com
e-mail: pcpanda@nitrkl.ac.in
Abstract—This paper presents a novel fuzzy control scheme On the other hand, the intelligent control that is based on
applied to shunt active power filters for harmonic and reactive artificial intelligence can emulate the human thinking process.
power compensation. A TSK type fuzzy logic controller is In the knowledge of expert that expressed in rules, fuzzy logic
proposed for APF reference current generation. To control
presents a slightly superior dynamic performance when
the maximum switching frequency of the converter within
compared with a more conventional scheme [11]. The
limit a novel fuzzy hysteresis band current controller is used.
The band height, based on fuzzy control principle is changed advantages of fuzzy logic controllers over conventional
with the value of supply voltage and slope of reference current. controllers are that they do not require an accurate
mathematical model, they can work with imprecise inputs, can
Keywords—active power filter, nonlinear load, TSK type fuzzy handle non-linearity and they are more robust than
controller, harmonics, indirect current control scheme conventional nonlinear controllers [8]. Among various PWM
techniques, hysteresis fixed band current control is popularly
I. INTRODUCTION used because of its simplicity of implementation. But this
Power electronics based non-linear loads such as power technique has the disadvantage of uncontrolled frequency which
converters in industrial applications, home appliances such as results in increased switching losses and excessive ripples in
TV sets, personal computers etc. are increasing in a never before source current. However, an adaptive hysteresis band current
rapid rate. These loads are known as generators of current control technique can be programmed as a function of active
harmonics and tend to distort the supply current. They are also filter and supply parameters to minimize the influence of current
responsible for low system efficiency, poor power factor, distortions on modulated waveform [10].
disturbance to other consumers and interference in nearby
communication networks. The concept of using active power II .PROPOSED CONTROL SCHEME
filters in order to compensate harmonic currents and reactive Fig.1 shows the active power filter compensation system
power of locally connected non-linear loads has been so far and Fig.2 shows the schematic diagram of fuzzy control scheme.
investigated and shown to be viable solution for power quality To implement the control algorithm in closed loop, the DC side
improvement [1,2]. APFs, may be classified into pure active filters capacitor voltage is sensed and compared with a reference value.
and hybrid active filters [4,5]. Hybrid APFs are primarily used for The error and integration of error are the inputs to the controller.
harmonic mitigation. With fast switching, low power loss power A TSK type fuzzy controller is used for this purpose as it is
electronic devices and fast digital signal processing devices shown in [9] that a TSK type fuzzy controller not only reduces
available at an affordable cost, it is feasible to embed a variety of the total harmonic distortion but also the settling time of DC
functions into a pure APF to make it a power quality conditioner capacitor is significantly decreased. The output of the controller
[6]. APF eliminates system harmonics by injecting a current to after a limit is the peak reference source current. This peak value
the system that is equal to the load harmonics. Since the load is multiplied by unit vectors in phase with the source voltages
harmonics to be compensated may be very complex and to obtain the reference source currents. These reference currents
changing rapidly and randomly, APF has to respond quickly and actual source currents are given to a hysteresis-based
and work with very high control accuracy in current tracking carrierless PWM converter. The difference to the reference current
[3].Two types of current control techniques, namely direct template and the actual current decides the operation of
and indirect current control have been discussed in [7]. It switches. Unlike conventional fixed band hysteresis controller,
has been found that the indirect current control technique an adaptive hysteresis band current control technique is used
which is based on sensing line current only is simpler, requires where the band height, based on fuzzy control principle is
less hardware and offers better performance. The scheme changed with the value of supply voltage and slope of reference
uses a conventional PI controller to obtain reference current current.
template. However, the design of PI controller requires precise
linear mathematical model of the system which is difficult to
obtain and may not give satisfactory performance under
parameter variations.
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2. ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 02, June 2011
A. DC voltage control
A TSK type fuzzy controller has been chosen for closed
loop control of DC voltage. The error x1(k)=Vdcref-Vdc and
integration of error x2(k)=”e(k) are used as input for fuzzy
processing. The TSK type fuzzy controller scheme structure is
given in Fig.3. The error and integration of error are partitioned
into two trapezoidal fuzzy sets P(positive) and N(negative) as
given in Fig.4. The values of L1 and L2 depend upon maximum
value of error and its integration.
The TSK fuzzy controller uses following four simplified rules.
R1) If x1(k) is P and x2(k) is P, then u1(k) =a1.x1(k)+a2.x2(k)
R2) If x1(k) is P and x2(k) is N, then u2(k) =K2u1(k)
R3) If x1(k) is N and x2(k) is P, then u3(k) =K3u1(k)
R4) If x1(k) is N and x2(k) is N, then u4(k) =K4u1(k)
In the above rules, u1, u2, u3, u4 represent the consequent of TS
fuzzy controller. Using Zadeh’s rules for AND operation and the
general defuzzifier, the output of the TS fuzzy controller is
Fig.1 Basic control Scheme
… (1)
Fig.2 Block diagram of proposed Scheme
(a) (b)
Fig.4 Membership functions for (a) x1 and (b) x2
However, for ã=1, we get the centroid defuzzifier with u(k) given
by
u(k)=a.x1(k)+b.x2(k) … (2)
where
a=a1K and b=a2K … (3)
and
Fig3. TSK Fuzzy control scheme with error and its integration
… (4)
The above TS fuzzy controller is a highly non-linear variable
gain controller and the coefficients a1, a2 produce wide variations
of controller gain. The values of G1, a1, a2, K2, K3, K4, L1, L2 are
given in the Appendix.
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3. ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 02, June 2011
B. Fuzzy hysteresis band current control in Fig.5. The rule is expressed in the form of IF (antecedent)
The most commonly used current control strategy is the THEN (consequence) form. Control rule table is given in Table.1.
fixed band hysteresis method. But it has the disadvantage of The centre-of-mass method is used for defuzzification.
uncontrollable high switching frequency. This high switching
frequency produces a great stress for the power transistors and III. SIMULATION RESULTS
induces important switching losses. To improve this control, an The system parameters selected for simulation studies are
adaptive hysteresis band current control technique can be Vm=100V, Rs=0.1&!, Ls=0.15mH, Rf=0.1&!, Lf=0.66mH, Vdcref=220V.
programmed as a function of the active filter and supply A three phase diode rectifier with R-L load is considered as a
parameters to minimize the influence of current distortions on non-linear load. Initially load parameters are taken as Ll=20mH
modulated waveform [10]. The hysteresis band is given by and Rl=6.7&!. The switch on response of the system is given in
Fig.6.
… (5)
j= 1, 2, 3; fm is the modulating frequency, is* is the reference
source current and dis* /dt represents its slope.
(Fig.6 Performance of the system for load of 6.7ohm &20mH)
Fig.5 Membership functions for input variables di s*/dt, vs(t) and
output variable HB
TABLE.1 CONTROL RULE TABLE
To improve active filter performance equation (5) is implemented
in our case with fuzzy logic. The supply voltage vs(t) and slope
of reference source current dis*/dt are taken as inputs for fuzzy
processing and output is HB. To construct a rule base, the inputs
Fig.6(e) Source current
are partitioned into five primary fuzzy sets labeled as {NL, NM,
Fig.6 Performance of the system for load of 6.7ohm &20mH
EZ, PM, PL}. Similarly the output variable HB is divided into five
fuzzy sets labeled as {PVS, PS, PM, PL, PVL}. A triangular
membership function has the advantages of simplicity and easy
to implement, hence therefore chosen for this application. The
normalized membership functions used for fuzzification are given
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4. ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 02, June 2011
Fig.7 (a) Harmonics distribution spectrum of Load current Fig.8 (d) Harmonic distribution spectrum of source current at t=0.5s
Fig.8 Dynamic Response of the system
System performance is analysed under dynamic conditions
also. At initial stage the load on rectifier is Rl=6.7ohm and
Ll=20mH. At time t=0.4s the resistance is increased from 6.7ohm
to 10 ohm. Again at time t=0.7s it is decreased to 6.7ohm. The
system responses are shown in Fig.8. It can been seen that the
settling time of DC capacitor is about one and half cycles. Also
the THD is only 1.50%.
Fig.7 (b) Harmonics distribution spectrum of Source current at t=0.3s IV. CONCLUSION
The total harmonic distortion (THD) of load current is
28.26%. After compensation THD of source current is reduced
From the simulation responses, it is evident that the reference
to 1.59%. Waveforms in Fig.6 clearly indicate that the harmonic
current generator and the adaptive hysteresis band current
component of load current is supplied by APF and the source
controller are performing satisfactorily. In all the cases studied
current is in phase with supply voltage. Also it has been observed
the total harmonic distortion is well below 5%, the harmonic limit
that the settling time of DC link capacitor voltage is about 1
imposed by IEEE-519 standard. Also the dynamic performance
cycle only.
of the system is impressive as the settling time of dc-link capacitor
voltage is within two cycles. This is quite important in the context
that at this condition, the real power balance between the source
and the load is realized.
V. APPENDIX
Parameters used in TSK fuzzy control scheme are G1=1/
Fig.8 (a) Load Current
4000; a1=1.1; a2=0.35; K2=-0.5; K3=1.8; K4=-0.08; L1=30; L2=10
Fig.8 (b) DC link Capacitor voltage
Fig.8 (c) Source current
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